The present invention relates to a gel coating resin composition having improved surface hardness, as well as high abrasion resistance, good workability, good film forming properties and strong adhesion to adjacent layers. The present invention also relates to a shaped article containing a cured layer of this gel coating resin composition. More particularly, the present invention relates to a shaped synthetic resin article having a surface layer that is gel coated from said resin composition and which is cured with an organic peroxide to produce improved surface hardness, as well as high resistance to abrasion, cigarette burns, and heat.
Gel coating resins are painted on the surface of a mold to produce surface layers on shaped articles. Usually, the surface of the resulting gel coating layer where it contacts air is lined either with a paint composition made of an unsaturated polyester resin and glass fibers so as to produce a shaped FRP (fiber-reinforced plastic) article or with a paint composition made of an unsaturated polyester resin and a filler to make a shaped plastic article such as simulated marble. The so produced shaped articles are primarily used for residential products such as dressing tables in washrooms, kitchen counters and bathtub enclosures. One problem with unsaturated polyester resins that are commonly used as gel coating resins is their low pencil hardness (.ltoreq.4H) which makes them highly susceptible to surface damage.
Hard coating agents based on such resins as fluorine, acrylic and silicone resins are commercially available as resins having high surface hardness but all of them are to be cured with UV radiation or electron beams and require post-painting after they are shaped in a desired form. If these resins are used as hard coating resins, expensive apparatuses are necessary for performing irradiation with UV light or electron beams and, at the same time, difficulty is involved in providing uniform radiation energy over the entire surface of shaped articles having many and various forms. A fatal defect with these resins is that prolonged cure time produces nonuniformity between curing on the surface where the coating contact the surface of a mold and curing on the surface of contact with air. In other words, the latter surface which is being irradiated with UV light or electron beams cures faster than the surface of contact with the mold, and this makes it impossible to ensure good adhesion to the subsequently applied lining of a forming resin material unless some preliminary treatment is performed.
Combinations of unsaturated polyesters and acrylic compounds are disclosed in Japanese Patent Application (OPI) Nos. 103994/76, 47039/85, 8318/86 and 69855/86 (the term "OPI" as used herein means an unexamined published Japanese patent application). However, the combination proposed in Japanese Patent Application (OPI) No. 103994/76 is not intended to be used as a gel coating resin, so the unsaturated polyesters considered to be suitable for use have low degrees of unsaturation and acrylic compounds having two unsaturated double bonds in one molecule are listed, with the combination of compounds that have one unsaturated double bond in one molecule being mentioned as a suitable example.
The composition disclosed in Japanese Patent Application (OPI) No. 103994/76 is freed of the odor characteristic of styrene by completely replacing styrene monomers with acrylic compounds. Furthermore, the acrylic compounds used have low viscosity in order to ensure good workability. However, in the absence of styrene monomers, this composition cures poorly and the resulting coating is not sufficiently thin and hard to provide a desired gel-coated surface layer.
The combination of unsaturated polyester and acrylic compound described in Japanese Patent Application (OPI) No. 47039/85 is to be used as a coating composition for shaped polystyrene foams, so it does not permit the use of polymerizable monomers that will dissolve or erode the shaped polystyrene foams. If styrene monomers are used in 15 to 60 parts by weight as in the present invention, the surface of the shaped polyurethane foam will be eroded. So, in order to avoid this problem, the prior patent states that styrene monomers can only be used in amounts of up to 10 parts by weight. Under these circumstances, the acrylic compound that can be used is limited to trimethyloloropane and other low-viscosity acrylic compounds having a functionality of no more than three. However, this does not ensure the production of a gel-coated layer of high hardness as in the present invention.
Japanese Patent Application (OPI) Nos. 69855/86 and 8318/86 disclose a process for producing a resin forming composition that is comprised of a polyfunctional acrylate based resin, a polyester resin and a filler, as well as a process for producing a shaped article having a surface of enhanced hardness, with a gel-coated layer being formed from said composition. However, the polyester resins disclosed in these prior patents are limited to either isophthalic acid containing polyester acrylate based resins or vinyl ester containing unsaturated polyester resins, and the polyfunctional acrylate based resins disclosed are also limited to those special acrylate resins which are curable with UV radiation. In other words, the above-mentioned prior patents do not disclose a gel coating resin composition composed of a specific unsaturated polyester, a specific polyfunctional acrylic monomer and specific copolymerizable ethylenically unsaturated monomer as in the present invention, nor do they disclose a shaped article having a gel coat made from this particular resin composition. As a further problem, compositions using polyester acrylate or vinyl ester based resins usually have dark colors on the surface when they are shaped, so the resulting shaped article will have a rather poor appearance. Furthermore, the poor thixotropy imparting and curing properties of such compositions present problems during shaping operations with respect to spraying characteristics and drying properties, thereby impairing the efficiency of shaping operations.